Anthrax, Megadeth, Metallica, and Slayer are the bands that make up the Big Four of Thrash Metal. The reason they are called the Big Four is because they pretty much influenced almost every metal band after them and put thrash metal on the map.

Their less-used name is the Four Greatest Bands Ever, but they are modest.

The Big Four of Thrash is Anthrax, Megadeth, Metallica, and Slayer, but there are plenty more good thrash bands like Exodus, Kreator, Testament, Dark Angel, and Overkill

Metallica, Exodus, Megadeth and Anthrax. Slayer is not one of the original top four, despite what you may hear. Top citation is Dave Mustaine who pushes a "top 5" that includes Slayer, but does not replace Exodus.

Metallica, Megadeth, Anthrax and Exodus are in the original Big Four of Thrash

Metallica, Megadeth, Anthrax, Exodus (not Slayer). As much as we might want it, Slayer was never considered one of the original big four (probably should have been a "big five"). I have seen Slayer replacing Exodus on several pages now including this one and Wikipedia. Not so. Slayer was often categorized differently than the mainstream 80's Thrash bands (Death metal, etc, although I don't think this exactly fits either).

The Big Four of Thrash from the 1980's were Metallica, Megadeth, Anthrax, and Exodus